1. Field of Invention
The invention relates generally to the dicing of semiconductor wafers and, more particularly to the monitoring of blade location and flange clearance for safely cutting a semiconductor wafer.
2. Background Art
Die separation, or dicing, by sawing is the process of cutting a microelectronic substrate into its individual circuit die with a rotating circular abrasive saw blade. This process has proven to be the most efficient and economical method in use today. It provides versatility in selection of depth and width (kerf) of cut, as well as selection of surface finish, and can be used to saw either partially or completely through a wafer or substrate.
Wafer dicing technology has progressed rapidly, and dicing is now a mandatory procedure in most front-end semiconductor packaging operations. It is used extensively for separation of die on silicon integrated circuit wafers. Increasing use of microelectronic technology in microwave and hybrid circuits, memories, computers, defense and medical electronics has created an array of new and difficult problems for the industry. More expensive and exotic materials, such as sapphire, garnet, alumina, ceramic, glass, quartz, ferrite, and other hard, brittle substrates, are being used. They are often combined to produce multiple layers of dissimilar materials, thus adding further to the dicing problems. The high cost of these substrates, together with the value of the circuits fabricated on them, makes it difficult to accept anything less than high yield at the die-separation phase.
Dicing semiconductor wafers by sawing is an abrasive machining process similar to grinding and cutoff operations that have been in use for decades. However, the size of the dicing blades used for die separation makes the process unique. Typically, the blade thickness ranges from 0.6 mils to 500 mils, and diamond particles (the hardest well known material) are used as the abrasive material ingredient. Because of the diamond dicing blade""s extreme fineness, compliance with a strict set of parameters is imperative, and even the slightest deviation from the norm could result in complete failure.
The diamond blade is a cutting tool in which each exposed diamond particle comprises a small cutting edge. Various dicing blades are available commercially. By way of example, a sintered diamond blade includes diamond particles which are fused into a soft metal such as brass or copper, or incorporated by means of a powdered metallurgical process; a plated diamond blade includes diamond particles which are held in a nickel bond produced by an electroplating process; and a resinoid diamond blade is one in which diamond particles are typically held in a resin bond to create a homogeneous matrix. Silicon wafer dicing typically uses the plated diamond blade, which has proven to be most successful for this application.
Because most state-of-the-art dicing equipment has been designed specifically to dice silicon wafers, problems arise when it is necessary to cut harder and/or more brittle materials. Blade speed and torque, depth of cut, feed rate, and other performance parameters have been optimized for silicon. However, hard and brittle materials require different blades and equipment operating parameters, the proper selection of which is a key to success for high-yield dicing. In any cutting operation, tool sharpness is of primary importance. More exactly, it is necessary that the cutting tool maintain its sharpness throughout the cutting operation. When cutting hard material such as sapphire or garnet, the cutting edges become dull quite rapidly. Because the dulled cutting edges cannot be re-sharpened in the usual manner, it is desirable that they be pulled loose from the blade, or else be fractured to expose new sharp cutting edges.
An important characteristic of the resinoid diamond blade that promotes effective cutting is its self-sharpening ability. The blade requires no dressing at all, in contrast to most metal-bonded (sintered or electroplated) diamond blades. Sharpening is accomplished automatically by the cutting process. As a cutting edge becomes dull, it experiences increased cutting forces that eventually either pull the diamond particle loose from the blade or else fracture it to produce a new sharp cutting edge. A diamond blade that does not exhibit this property cannot properly cut hard materials, nor can it perform properly if saw operating parameters interfere with the self-sharpening mechanism.
By way of example, U.S. Pat. No. 4,219,004 addresses a problem in the art of getting the blade cutting surface perpendicular to the substrate being cut and discloses blade mounting means comprising a pair of generally flat round collars, flanges, having a round central opening for receipt by the saw spindle. Further, the outer diameters of the collars are less than the blade diameter for providing an exposure of approximately 15 mils. A blade exposure not greater than 20 to 25 times the blade thickness is recommended. Replacing the collars with those having smaller diameter are disclosed for providing desired exposure and for replacing collars as the blade wears and exposure is reduced. Methods for monitoring or measuring the exposure during the dicing of the substrate is not addressed. U.S. Pat. No. 4,787,362 discloses an abrasive cutting blade having very high rigidity useful dicing silicon wafers and hard materials. The use of the flange or spacer for maintaining blade rigidity and providing blade exposure sufficient for completely penetrating the work piece and cutting partially into the intermediate carrier typically used is disclosed. Wobble or run-out is of concern and is inversely proportional to the blade exposure. As a result, blade exposure is held to tight and typically minimal dimensions. A rigid blade core is described for preventing run-out from causing the core to make contact with the workpiece and causing widening of the cut and a less than even cut. Making the flange larger for providing less exposure is not addressed. However, less exposure means greater chance for inadequate cooling and greater chance of the flange hitting the work piece. There remains a need to effectively and economically resolve these problems. U.S. Pat. No. 3,987,670 discloses a displacement transducer manually applied to a diamond blade cutting surface for measuring a distance from the blade cutting edge to a fixed reference distance on the blade. The transducer is mounted on a portable fixture. Blade wear of diamond blades generally in the range of 18 to 36 inches are addressed and the problems associated with measuring blade wear of these blades are identified. The transducer is provided with suitable readout devices to determine blade wear. Although blade wear is addressed, it is for relatively large, easily visible blade sizes, and measured while the blade is held motionless. Further, the issues associated with exposure and depth of cut into a substrate is not addressed. Flange clearance is not a major concern for 18xe2x80x3 to 36xe2x80x3 blades.
There is a need to monitor blade exposure, the amount of blade extending from the flanges holding the blade therebetween, during a wafer or substrate dicing for maintaining sufficient clearance between the flange edges and the substrate to provide adequate cooling, and further for preventing the flanges from contacting the substrate, often containing electronic chips valued in the many thousands of dollars. There is further a need to monitor and control the location of the cutting blade with respect to the location of the wafer to be cut and to efficiently and effectively control positions prior to a first cut and during movement of the wafer on its table for subsequent cuts. By way of example, a dicing machine user will typically try to mount the wafer at the center of the table or chuck holding the wafer during the cutting operation. In the alternative, computer aided chuck and saw movement will determine measured cuts from the table center and move the dicing saw relative to the center coordinates, sometimes actually moving the table to the center prior to moving it to the appropriate cutting location. This adds expensive operating time, especially when one considers that thousands of cuts may be required within one wafer dicing project. When a cut is to be made close to an edge of the wafer, and the blade is allowed to make a cut close to the wafer edge, the blade may ripe off a section of the wafer, which can require disposal of the entire wafer, or extensive attempts and time for salvaging what is typically a very expensive wafer including multiple electronic elements.
Various approaches have been used to identify a locations of a workpiece in computer aided machines. By way of example, U.S. Pat. No. 4,233,625 to Altman discloses the use of television monitoring for aligning successive configurations of semiconductors. U.S. Pat. No. 5,422,579 to Yamaguchi discloses the use of a camera for identifying probe positions on a card and recognizing reference probes for providing a corrective movement to a work table. U.S. Pat. No. 4,819,167 to Cheng et al. discloses a system and method for determining the location of a semiconductor wafer relative to its destination position using an array of optical sensors positioned along an axis transverse the path of movement of the wafer. Trigger points provided by the sensor array as the wafer is moved, provide locus information data to a processor for calculating the center of the wafer. U.S. Pat. No. 3,670,153 to Rempert et al. discloses the use of a light sensing element and scanning of the object for detecting dark and light regions in determining edges of the object to be measured. In spite of the many computerized optical devices and configurations, there still remains a need to economically provide a method for effectively and efficiently locating the position of the wafer on the work table for optimizing movement of the table or workpiece during sawing operations and for providing a safe location at which the saw can operate without damage to the wafer and saw, or hazard to the saw and operator.
In view of the foregoing background, it is therefore an object of the present invention to provide a method for safely and efficiently dicing a semiconductor wafer or substrate by moving the table relative to the saw based of a location of the wafer, while preventing the blade flange from contacting the substrate. It is further an object of the invention to prevent the cutting of a substrate so close to its edge that it may shatter the substrate or damage the blade. It is yet another object of the invention to monitor flange clearance during the cutting of the wafer for cutting the wafer without having the flange contact the wafer as a result of blade wear. It is yet another object of the invention to provide automation to the traditionally manual and semiautomatic monitoring of the wafer dicing process.
These and other objects, features, and advantages of the invention, are provided by a method for dicing a substrate using a programmable dicing saw. The dicing saw includes a processor operable for movement of a spindle carrying a dicing blade and a work surface upon which the substrate is removably secured. Movement of the dicing blade toward and away from the work surface is controlled by movements within an orthogonal coordinate system having its center at a center location of the work surface. The dicing blade is mounted onto the dicing saw spindle juxtaposed between a flange pair for rotation of the dicing blade about a spindle axis. The dicing blade has an outer diameter defining a cutting edge and is greater than each flange diameter of the flange pair for providing a blade exposure for cutting into a substrate. Preferably, the substrate to be cut is removably securing onto the work surface and within a blade path of the dicing blade.
Locating the center of the substrate provides for an efficient movement of the blade relative to the center and save time when compared to attempts to manually center the substrate and move the blade during the cutting process relative to the center of the work surface rather than the center of the substrate. A preferred method of locating the center of the blade includes the steps of aligning the dicing blade with a first edge of the substrate for determining a substrate first edge location on the work surface, aligning the dicing blade with a second edge of the substrate for determining a substrate second edge location on the work surface, wherein the first edge laterally opposes the second edge and the rotational axis of the dicing blade is perpendicular to the blade paths along the first and second edges, rotating the substrate ninety degrees about an axis perpendicular thereto, aligning the dicing blade with a third edge of the rotated substrate for determining a substrate third edge location on the work surface, wherein the blade path along the third edge is perpendicular to the blade path along the first edge, and aligning the dicing blade with a fourth edge of the substrate for determining a substrate fourth edge location on the work surface, wherein the third edge laterally opposes the fourth edge. Edge data representative of the measured substrate first, second, third, and fourth edge locations is entered into the processor operable with the dicing saw for determining the center of the substrate and calculating a distance between the center of the substrate and the center of the work surface for providing a compensating command to the programmable dicing saw. Movement of the substrate is then made relative to the center of the substrate. If desired, the dicing saw is located over the center of the substrate when initiating blade and work surface movement. The spindle and work surface are moved relative to the center of the substrate for positioning the dicing blade based on the compensating command.
In a preferred operation of the dicing saw when cutting a substrate includes aligning the dicing blade for making a cut into the substrate along a first blade path, dicing the substrate along the first blade path, and subsequent blade paths as desired. The blade outer diameter reduces with each cut into the substrate, thus reducing the blade exposure, and further reducing a clearance between the flange pair and a substrate top surface for each subsequent cut. Therefore, the flange clearance is determined and monitored by measuring the blade exposure after a preselected number of cuts during the substrate dicing.
In one preferred method when aligning the blade for dicing along an edge of the substrate, the dicing blade is first aligned for travel parallel to and proximate the first edge of the substrate. An offset command is provided to the programmable dicing saw for laterally moving the blade toward the center of the substrate prior to making a cut into the substrate. The offset command is representative of a preselected offsetting displacement of the blade from the edge to avoid damage to the blade and substrate that typically results when the blade slides along the substrate edge rather than cutting into the substrate.
The present invention provides for accurately making arcuate cuts into the substrate. With such a method, the dicing blade aligning comprises first aligning the dicing blade for travel along a first blade path at a preselected distance from the center of the substrate for the dicing thereof. A desired cut is made into the substrate. Then the dicing blade is aligned for travel along a second blade path at the preselected distance from the center of the substrate for the dicing thereof, wherein the second blade path radially opposes the first blade path. A desired cut is then made. The substrate is rotated by a preselected arc and the aligning and dicing steps are repeated for providing multiple cuts within the substrate. The substrate rotating comprises incrementally rotating of the substrate a multiplicity of times sufficient for providing an arcuate cut to the substrate. With such a method, a circular shape can result.
To guard against damage to the substrate, the dicing method further comprises the step of automatically stopping the dicing of the substrate when the flange clearance is reduced to less than a preselected minimum clearance. A separation distance between the work surface and the blade cutting edge is calculated using the processor, and blade movement into the substrate is automatically stopped when the blade cutting edge is greater than a preselected separation distance. In one embodiment, the flange clearance is calculated by sensing the blade cutting edge during blade rotation and prior to the substrate cutting step for setting a reference position for the blade edge and spindle axis, and sensing the blade cutting edge after the preselected number of cuts for determining an axis position difference for the worn blade. The difference is used to update data input to the processor regarding the reduction in blade diameter, the blade exposure, and thus the step of determining the flange clearance. The blade exposure measurement is made at preselected intervals throughout the substrate dicing steps. The flange clearance is automatically calculated at preselected intervals throughout the substrate cutting. A minimum flange clearance is preselected for continuing the dicing. The minimum flange clearance should provide effective coolant flow to the blade, adequate blade rigidity and thus a squareness of cut, and an acceptable blade chipping. Calculating blade exposure includes measuring blade wear after a preselected number of cuts for automatically monitoring the exposure during the dicing step and providing a first stop movement signal to the processor when a minimum exposure results in a minimum flange clearance for the dicing steps. Calculating a separation distance between the work surface and the blade cutting edge is made and provides a second stop movement signal to the processor when a preselected maximum separation distance is measured, thus indicating blade wear. Movement of the dicing blade toward the work surface is automatically stopped when any stop movement signal is received. The exposure calculating step comprises the sensing of the blade edge during blade rotation prior to the dicing of the substrate for setting a reference position for the blade edge and spindle axis, and sensing the blade edge after the preselected number of cuts for determining an axis position difference for the worn blade. The exposure calculating step is made by the processor using the axis position difference and the flange diameter.